Well logging instruments that utilize radiation generators, such as neutron generators, have proven incredibly useful in formation evaluation. Such a neutron generator may include an ion source and a target. An electric field is generated within the neutron generator that accelerates ions toward the target at a speed sufficient such that, when the ions are stopped by the target, neutrons are generated and directed into a formation into which the neutron generator is placed. The neutrons interact with atoms in the formation, and those interactions can be detected and analyzed in order to determine various pieces of information about the formation.
These neutron generators are not without drawbacks, however. For example, certain cathodes used in ion sources emit not only electrons, but also electrically conductive particles. These electrically conductive particles may build up on insulating surfaces inside the neutron generator, thereby changing the characteristics of those insulating surfaces. This may adversely affect the electric field inside the neutron generator, and consequently alter the focal point of the ion beam, which may result in the ion beam not striking the intended portion of the target. In addition, this may cause insulators to fail, such that electrical arcing occurs between electrodes. The foregoing degrades the performance of the neutron generator, and thus the performance of the well logging instrument utilizing the neutron generator.
In addition, such cathodes used in ion sources utilize ohmic heating in their operation. While this provides stable and copious electron generation, it also consumes an undesirably large amount of power for some applications. For example, in logging while drilling (LWD) applications, downhole power is often provided by a mud turbine, which while convenient, does not produce a large amount of power, and as such, power savings in such applications are highly desirable. In slickline applications, downhole power is often provided by a battery, and thus power savings are highly desirable as well, since the more power consumed in such applications, the shorter amount of time the instrument can be operated downhole.
As such, further advances in the area of ion sources are desirable. It is desired for such new ion sources to emit fewer electrically conductive particles, and to consume less power than ion sources utilizing ohmically heated cathodes.